Laps used in opthalmic lens making have a surface curvature (typically either spherical or else toroidal-- the latter often called cylindrical) which lap curvature is reproduced or transferred to the surface of a lens blank by rubbing the face of the lap against the lens piece until the lens is worn down to exact conformity with the configuration of the lap, such process taking place in the presence of an intervening film of flowable abrasive material.
Such laps are initially obtained by starting with a small number of cast iron laps, each of a considerably different curvature. A set of identical models of each such type are then shaped to an individually different but related curvature by means of a lap shaping machine. Thus, starting with say 24 different casting types, a final total of say 2400 different laps may be provided, each different to some extent from the other and each thus capable of producing a different lens. After such finished lap is used on five or six successive lens blanks, it is usually resurfaced by the same lap shaping machine. By the past shaping machines, the work on a raw casting may require 30 minutes; resurfacing may be done in about five minutes.
Conventional lap shaping machines are based on oscillating a narrow, slanted, chisel-like scraper which is moved in an arcuate path across the lap face, with the lap itself being moved perpendicular thereto. Correlation of the two movements may be by worm gear and ratchet mechanism or the like; the length of stroke is determined by eccentric means manually set for each job, and the whole operated by a source of rotary power which can even be provided by a hand crank. Such devices are satisfactory only because they have been used for many years and nothing better is available for contrast. In particular, since cross strokes of the cutter-chisel are typically effected by an eccentric drive, the cutter speed across the stroke length is non-uniform and varies with the changing position of the eccentric mechanism; also, and angularly disposed point cutter functions only in one direction, that is on the forward half of its cycle.